The present invention relates to probe measurement systems for measuring the electrical characteristics of integrated circuits and other electronic devices operated at high frequencies.
There are many types of probing assemblies that have been developed for the measuring the characteristics of integrated circuits and other forms of microelectronic devices. One representative type of assembly uses a circuit card on the upper side of which are formed elongate conductive traces that serve as signal and ground lines. A central opening is formed in the card, and a needle-like probe tip is attached to the end of each signal trace adjacent the opening so that a radially extending array of downwardly converging needle-like tips is presented by the assembly for selective connection with the closely spaced pads of the microelectronic device being tested. A probe assembly of this type is shown, for example, in Harmon, U.S. Pat. No. 3,445,770. This type of probe does not exhibit resistive characteristics over a broad frequency range. At higher frequencies, including microwave frequencies in the gigahertz range, the needle-like tips act as inductive elements and the inductance is not counteracted by a capacitive effect from adjoining elements. Accordingly, a probing assembly of this type is unsuitable for use at microwave frequencies due to the high levels of signal reflection and substantial inductive losses that occur at the needle-like probe tips.
In order to obtain device measurements at somewhat higher frequencies than are possible with the basic probe card system described above, various related probing systems have been developed. Such systems are shown, for example, in Evans, U.S. Pat. No. 3,849,728; Kikuchi, Japanese Publication No. 1-209,380; Sang et al., U.S. Pat. No. 4,749,942; Lao et al., U.S. Pat. No. 4,593,243; and Shahriary, U.S. Pat. No. 4,727,319. Yet another related system is shown in Kawanabe, Japanese Publication No. 60-223,138 which describes a probe assembly having needle-like tips where the tips extend from a coaxial cable-like structure instead of a probe card. A common feature of each of these systems is that the length of the isolated portion of each needle-like probe tip is limited to the region immediately surrounding the device-under-test in order to minimize the region of discontinuity and the amount of inductive loss. However, this approach has resulted in only limited improvement in higher frequency performance due to various practical limitations in the construction of these types of probes. In Lao et al., for example, the length of each needle-like tip is minimized by using a wide conductive blade to span the distance between each tip and the supporting probe card, and these blades, in turn, are designed to be arranged relative to each other so as to form stripline type, transmission line structures. As a practical matter, however, it is difficult to join the thin vertical edge of each blade to the corresponding trace on the card while maintaining the precise face-to-face spacing between the blades and the correct pitch between the ends of the needle-like probe tips.
One type of probing assembly that is capable of providing a controlled-impedance low-loss path between its input terminal and the probe tips is shown in Lockwood et al., U.S. Pat. No. 4,697,143. In Lockwood et al., a ground-signal-ground arrangement of strip-like conductive traces is formed on the underside of an alumina substrate so as to form a coplanar transmission line on the substrate. At one end, each associated pair of ground traces and the corresponding interposed signal trace are connected to the outer conductor and the center conductor, respectively, of a coaxial cable connector. At the other end of these traces, areas of wear-resistant conductive material are provided in order to reliably establish electrical connection with the respective pads of the device to be tested. Layers of microwave absorbing material, typically containing ferrite, are mounted about the substrate to absorb spurious microwave energy over a major portion of the length of each ground-signal-ground trace pattern. In accordance with this type of construction, a controlled high-frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device under test. Broadband signals that are within the range, for example, of DC to 18 gigahertz (GHz) can travel with little loss from one end of the probe assembly to another along the coplanar transmission line formed by each ground-signal-ground trace pattern. The probing assembly shown in Lockwood et al. fails to provide satisfactory electrical performance at higher microwave frequencies and there is a need in microwave probing technology for compliance to adjust for uneven probing pads.
Several high-frequency probing assemblies have been developed for improved spatial conformance between the tip conductors of the probe and an array of non-planar probe pads or surfaces. Such assemblies are described, for example, in Drake et al., U.S. Pat. No. 4,894,612; Coberly et al., U.S. Pat. No. 4,116,523; and Boll et al., U.S. Pat. No. 4,871,964. The Drake et al. probing assembly includes a substrate on the underside of which are formed a plurality of conductive traces which collectively form a coplanar transmission line. However, in one embodiment shown in Drake et al., the tip end of the substrate is notched so that each trace extends to the end of a separate tooth and the substrate is made of moderately flexible non-ceramic material. The moderately flexible substrate permits limited independent flexure of each tooth relative to the other teeth so as to enable spatial conformance of the trace ends to slightly non-planar contact surfaces on a device-under-test. However, the Drake et al. probing assembly has insufficient performance at high frequencies.
With respect to the probing assembly shown in Boll et al., as cited above, the ground conductors comprise a pair of leaf-spring members the rear portions of which are received into diametrically opposite slots formed on the end of a miniature coaxial cable and which are electrically connected with the cylindrical outer conductor of that cable. The center conductor of the cable is extended beyond the end of the cable (i.e., as defined by the ends of the outer conductor and the inner dielectric) and is gradually tapered to form a pin-like member having a rounded point. In accordance with this construction, the pin-like extension of the center conductor is disposed in a spaced apart, generally centered position between the respective forward portions of the leaf-spring members and thereby forms, in combination with these leaf-spring members, a rough approximation to a ground-signal-ground coplanar transmission line structure. The advantage of this particular construction is that the pin-like extension of the cable's center conductor and the respective forward portions of the leaf-spring members are each movable independently of each other so that the ends of these respective members are able to establish spatially conforming contact with any non-planar contact areas on a device being tested. On the other hand, the transverse-spacing between the pin-like member and the respective leaf-spring members will vary depending on how forcefully the ends of these members are urged against the contact pads of the device-under-test. In other words, the transmission characteristic of this probing structure, which is dependent on the spacing between the respective tip members, will vary in an ill-defined manner during each probing cycle, especially at high microwave frequencies.
Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor. A tip section of the microwave probe includes a central signal conductor and one or more ground conductors, generally arranged in parallel relationship to each other along a common plane with the central signal conductor, to form a controlled impedance structure. The signal conductor is electrically connected to the inner conductor and the ground conductors are electrically connected to the outer conductor of the coaxial cable. A shield member is interconnected to the ground conductors and covers at least a portion of the signal conductor on the bottom side of the tip section. The shield member is tapered toward the tips with an opening for the tips of the conductive fingers. The signal conductor and the ground conductors each have an end portion extending beyond the shield member and, despite the presence of the shielding member, the end portions are able to resiliently flex relative to each other and away from their common plane so as to permit probing of devices having non-planar surfaces.
In another embodiment, Burr et al. disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor. A tip section of the microwave probe includes a signal line extending along the top side of a dielectric substrate connecting a probe finger with the inner conductor. A metallic shield may be affixed to the underside of the dielectric substrate and is electrically coupled to the outer metallic conductor. Ground-connected fingers are placed adjacent the signal line conductors and are connected to the metallic shield by way of vias through the dielectric substrate. The signal conductor is electrically connected to the inner conductor and the ground plane is electrically connected to the outer conductor. The signal conductor and the ground conductor fingers (connected to the shield by vias) each have an end portion extending beyond the shield member and, despite the presence of the shielding member, the end portions are able to resiliently flex relative to each other and away from their common plane so as to permit devices having non-planar surfaces to be probed. While the structures disclosed by Burr et al. are intended to provide uniform results over a wide frequency range, they unfortunately tend to have non-uniform response characteristics at high microwave frequencies.
Gleason et al., U.S. Pat. No. 6,815,963 B2, disclose a probe comprising a dielectric substrate that is attached to a shelf cut in the underside of a coaxial cable. The substrate projects beyond the end of the cable in the direction of the longitudinal axis of the cable. A signal trace for conducting a test signal between the center conductor of the coaxial cable and a probing or contact pad on the device under test (DUT) is formed on the upper side of the substrate. At the distal end of the signal trace, near the distal edge of the substrate, a via, passing through the substrate, conductively connects the signal trace to a contact bump or tip that will be brought into contact with the contact pad of the DUT during probing. A conductive shield which is preferably planar in nature is fixed to the bottom surface of the substrate and electrically connected to the outer conductor of the coaxial cable. The conductive shield is typically coextensive with the lower surface of the substrate with the exception of an aperture encircling the contact tip for the signal trace. Contact tips may also be provided for contacting ground contact pads spaced to either side of the signal probe pad on the DUT. Compared to coplanar type probes, this probe tip provides superior electromagnetic field confinement and reduces unwanted coupling or cross talk between the probe's tips and with adjacent devices. However, at high frequencies, approximately 220 GHz and greater, the length of the conductive interconnection between the probe tip and the coaxial cable connection becomes a significant fraction of the wavelength of the signal and the interconnection acts increasingly as an antenna, emitting increasingly stronger electromagnetic fields that produce undesirable coupling paths to adjacent devices. In addition, the conductive interconnection comprises a single metal layer deposited on the dielectric substrate and the relatively small section of the conductive interconnection limits the current carrying capacity of the probe.
What is desired, therefore, is a probe tip for an on-wafer probe enabling probing at higher frequencies, reducing stray electromagnetic fields in the vicinity of the probe tip to reduce cross talk with adjacent devices and capable of conducting a substantial current.